Even though it has been generally recognized for a number of years that it would be desirable to provide a computer keyboard in which the keystroke force required to activate a keyswitch is adjustable, it has been found that to include such a feature substantially increases the cost of the keyboard and may reduce other performance characteristics of the keyboard, including its useful life.
For example, U.S. Pat. No. 5,466,901 granted to Isao Mochizuki on Nov. 14, 1995 is directed to an "adjustable touch computer keyboard" having scissor-like leg structures supporting keytops with compression coil springs used to assist in biasing the keytops to the "up" position. A slide mechanism is used to adjust the compression of the spring to vary the "touch" or "feel" of the key. Such a design is quite expensive.
Another example is described in U.S. Pat. No. 4,500,758, granted to Peter U. Guckenheimer on Feb. 19, 1985. Such patent is directed to a keyboard having a mechanical cam means to adjust the length of the keystroke to vary the "tactile feel" of the keys. U.S. Pat. No. 5,220,318 granted to Darrell S. Staley on Jun. 15, 1993 describes an adjustable "touch" control using magnetic key plungers located within adjustable magnetic fields to vary the forces required to depress the keys and activate the keyswitches.
U.S. Pat. No. 4,795,888 granted to Andrew R. MacFarlane on Jan. 3, 1989 describes a computer keyboard that has a variable force keystroke feature in which an apertured air pressure bladder is placed underneath the keytops so that as the keytops are depressed the outer edges of the keytops engage the upper layer of the bladder thereby increasing the force required to continue depressing the keytop until the keyswitch is actuated. The air pressure in the bladder may be adjusted to vary the keystroke force required to actuate the keyswitch. One of the major problems with such a design is that the spring action of the air pressure is not linear over the full stroke of the key but rather is more exponential in character, thereby not only changing the force required to depress the keytop but also dramatically changing the "tactile feel" (shifting the force-displacement curve laterally) at the various air pressure settings. Furthermore, the force required to depress a key will vary depending upon the size of the keytop. The larger keytops will require a larger force to depress, whereas the smaller keytops will require a smaller force to depress.
One of the objects and advantages of this invention is to provide a computer keyboard that overcomes many of the problems that are identified above and to provide a less costly and more reliable adjustable force keystroke feature to enable users to more readily adjust the keystroke force to "customize" the keyboard to their preferences without dramatically changing the fundamental "tactile feel" characteristics of the keyboard.